Hesitation, Ignition Miss 2001 Corolla S

Hope someone here can help me to scope out a problem with our 2001 Corolla S. Car has 135K miles and has given us little problem in the past.

A couple of months ago, we started getting a bit of a miss while it was idling - nothing much, acted like it needed new plugs. However, it also had developed a bit of a ping during freeway ramp acceleration.

I started doing work on the vehicle. Changed oxygen sensors (both upstream and downstream) with NGK (upstream) and Denso (downstream) units. Also installed new NGK platinum plugs at the same time.
After I did all this work, it not only had MORE of a miss, it had a hesitation starting from a dead stop particularly when "cold", along with a too low idle speed, again particularly when "cold".

Took car to Toyota independent mechanic that we've used several times. He cleaned the throttle body (do not know if he cleaned the MAF, though). Car acted quite a bit better after the MAF cleaning. He got it to duplicate the problems, but didn't have any ideas as to the solution since he said it was "inconsistent". (Yes, it IS inconsistent when warmed up). He suggested putting a bottle of Techron in it, which I did followed by filling it up with gas. The Techron didn't really change anything.

After researching the issue, I tried two more things - cleaned the PCV valve and the OCV filter. I cleaned these at separate times, and each time, the car got a little better. The only incident I had when cleaning the OCV filter was that I smashed it a bit on the metal part (but did not bend or break the plastic frame).

Any suggestions anyone? A family friend is suggesting that we have an "air leak" somewhere, perhaps an injector seal or a vacuum hose leaking. We're at our wits end on the car right now. Thanks.

Couple of misses here or there, or a pretty consistent string of misses?

Have you tried the disconnecting the spark plugs, one at a time, to see if there is any one cylinder that seems to be the culprit? The cylinder that you disconnect that doesn't have a huge change in engine idle is the cylinder that is giving you problems.

Same can be done on the OCV valve - disconnect it and see if the engine runs any differently. OCV is OK if the car doesn't run differently if disconnected at idle.

Deformation in the OCV filter is OK, as long as the plastic "cage" is not damaged. When you cleaned it, was it pretty clogged up, just a little dirty?

Assuming that you disconnected the battery when you replaced the O2 sensors? Usually good practice to do so when working around anything electrical, unless you are debugging that component. Disconnecting the battery also clears the ECM settings, allow the car to relearn new running parameters. It will run a bit "off" after all the work you've done, but should be good to go after a single drive cycle.

Other possibilities:

- leaking or bad fuel injector (could be at the O-rings, sucking in air)
- vacuum leak, concentrate around the induction system (intake, throttlebody, injectors, etc.) - this can be pretty tough to find, but likely if there is a difference in idle quality from a cold soak to completely warm engine.
- exhaust leak, more so since there was a marked change in behavior after you changed the O2 sensors. If you have the old ones, might be worth a shot to have them tested, or even try swapping them back for testing.
- faulty throttlebody position sensor (TPS)
- faulty camshaft position sensor
- faulty VVTi (OCV, VVTi actuator)
- chassis grounding issue
- faulty coil on plug
- could be also a bad plug (gap is wrong), bad O2 sensor (I've has some bad out of the box)
- verify wiring connections are tight, wiring has no damage to insulation/connector/etc.

Thanks very much for all your help. This may be a gas-related issue, as well. We've been using a chain convenience store's (Quik Trip) gasoline for quite some time, but recently, there have been rumors of them having > 10% ethanol. The problems are greatly improved by switching brands.